Recharging of discharged hearing aid batteries is a relatively difficult task because of their small size, precise and narrow recharge current and voltage limits, and the fact that single recharge economies achieved are small in magnitude, so that expensive recharging equipment is simply not justified. In fact, most hearing aid battery manufacturers label their products as "non-rechargeable".
Several hearing aid battery chargers have been devised which utilize 110 v. a-c domestic power supply circuits, such as shown in U.S. Pat. No. 3,297,933, but these are expensive in first cost and require underwriters' approval, so that they are not very practical.
Thus, there is a real need for a low cost recharging apparatus which will function satisfactorily from a low voltage dry cell source and which will recharge hearing aid batteries repetitively a large number of times without damaging them, and also will be small in size and so stable in operating characteristics that they can be adjusted permanently in the factory before sale, after which no adjustments whatever will be required on the part of the purchasers.